Seven months ago I wrote a column about the paradoxical arithmetic of mass testing for a rare disease, showing that even very accurate tests are wrong more often than we think.

It was a fun intellectual exercise thrown together as a last-minute replacement for a column that fell through. It got some reaction from intrigued readers but that’s about it, and we all moved on.

Then SARS-CoV2 moved in. Suddenly the use of widespread medical testing is one of the central issues of life.

So I thought I’d revisit the column and take it slightly further because the original included a simplification. A warning: You’re going to have to keep the terms “sensitivity” and “specificity” straight, which I never can.

First, a quick reminder of the paradox.

Let’s say we test 1 million New Hampshire residents for COVID-19 – roughly everybody over age 18. And let’s say 4% of people have the disease, which is the percentage of tests which have been turning up positive so far in New Hampshire. We use a test that is 95% accurate, which is a very good test.

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